The invention relates to a process for separating NH3 from a mixture containing NH3, CO2 and H2O which comprises an NH3 rectification step carried out in an NH3 separation device to which one or more streams containing NH3, CO2 and H2O, including the mixture, are supplied, with a stream consisting substantially of gaseous NH3 being formed in the NH3 separation device, separated from the mixture and removed.
Such a process is known from NL 7804668 A. In the known process, which can find application in processes for the preparation of melamine or urea or the combined preparation of melamine and urea, the mixture supplied to the NH3 separation device is gaseous or liquid. The NH3 separation device is designed as a distillation device; the energy requirement is met by means of steam. The gaseous NH3 stream that s formed comprises more than 95 wt % NH3 and comprises further inert gases. The gaseous NH3 stream does not contain CO2. The gaseous NH3 stream is partly condensed, with liquid NH3 being formed. The liquid NH3 is returned to the NH3 separation device. The remaining mixture is discharged from the NH3 separation device. In subsequent steps a stream consisting substantially of CO2 and a stream consisting substantially of H2O can be separated from the mixture.
A disadvantage of the known process is that the NH3 separation device is difficult to control. The composition, pressure and temperature are such that normal fluctuations in the process operation entail the danger of solids being formed. If this takes place, the solids must be removed by flushing with water, as a result of which the efficiency of the NH3 separation device decreases.
It is the object of the invention to reduce said disadvantage.
Said object is achieved in the process according to the invention in that a condensation step is carried out on at least one of the stream consisting substantially of gaseous NH3 or the one or more streams containing NH3, CO2 and H2O that are supplied to the NH3 separation device, with at least a part of the CO2 that is present being converted to a liquid phase.
An advantage of the process according to the invention is that the danger of solids being formed in the NH3 separation device is less than in the known NH3 separation device. This reduces the need for efficiency-impairing measures such as the said water flushing operation, so that the process is more stable at a lower consumption of energy, for example in the form of steam, and as a result is cheaper.
Without intending to give a theoretical explanation of the advantages of the process according to the invention, it is being assumed that the conversion to a liquid phase of CO2 has such an effect on the ratios in the NH3 rectification step between NH3, CO2 and H2O and/or the phase in which they are present that there is less danger of solids being formed. In addition, it is being assumed that it is possible to enlarge the operational possibilities of composition, pressure and temperature in the NH3 separation device in such a way that there is less danger of solids being formed.
The process according to the invention is applied to a mixture containing NH3, CO2 and H2O. The ratios between NH3, CO2 and H2O can vary within wide limits, as can the pressure and the temperature of the mixture. Preferably no solid material is present in the mixture. In addition, the way in which the NH3 rectification step to be discussed hereinafter is carried out may affect the possible ratios between NH3, CO2 and H2O, such as in the case of application of distillation in the NH3 rectification step. In that case it is important, as for example cited in NL 7804668 A, to take into account the known azeotropic nature of mixtures of NH3, CO2 and H2O. As a result, at a given composition and pressure only pure NH3 can be separated using ordinary distillation if the composition is in the so-called NH3-rich range, i.e. range I in FIG. 1 of NL 7804668 A. Analogously, only pure CO2 can be separated using ordinary distillation if the composition is in the CO2-rich range, i.e. range II in FIG. 1 of NL 7804668 A.
If the mixture is present in, or originates from, processes known per se for the preparation of melamine or urea, the mixture generally contains between 20 and 70% NH3, between 10 and 50% CO2 and between 10 and 70% H2O. Preferably the mixture contains between 25 and 60% NH3, between 15 and 40% CO2 and between 20 and 55% H2O. More preferably the mixture contains between 30 and 50% NH3, between 15 and 25% CO2 and between 25 and 50% H2O. Unless stated otherwise, said percentages here and hereinafter are weight percentages.
In the process according to the invention a NH3 rectification step is understood to mean a step, applied to the mixture, in which separation technology is used to form a stream that consists substantially of NH3. In principle every separation technology is suitable which ensures that the stream consisting substantially of gaseous NH3 is formed, separated from the mixture and can be discharged. Examples of possible separation technologies are membrane separation and distillation. Preferably distillation is applied.
It can be useful or necessary for one or more additional streams to be supplied to the NH3 rectification step that influence the thermodynamic equilibrium. An additional stream can also be supplied with the aim of separating NH3 from it, as in the case of the mixture. The additional streams can contain NH3 and/or CO2 and/or H2O. Examples of additional streams are liquid NH3 and recirculation streams from further process steps applied to the mixture. The NH3 rectification step is carried out in an NH3 separation device. If distillation is chosen as the separation technology, the pressures usually lie between 0.1 and 6 MPa, preferably between 0.3 and 4 MPa, more preferably between 0.6 and 3 MPa; the temperature usually lies between 5 and 160° C.
The stream consisting substantially of gaseous NH3 that is formed in the NH3 separation device and separated from the mixture is discharged. Besides NH3 said stream may also contain small quantities of other compounds such as CO2 and H2O; preferably the stream consisting substantially of gaseous NH3 contains less than 15% other compounds, more preferably less than 10%, even more preferably less than 8%, and most preferably less than 5% or even less than 1%. The separation effort required to further reduce the quantity of other compounds can be weighed against the quantity of the other compounds that is allowable in the light of further application of the stream consisting substantially of gaseous NH3. In addition, if allowing a certain quantity of CO2 in the stream consisting substantially of gaseous NH3, for example 5% or less, leads to a simplification or stabilization of the operation of the NH3 separation device, it is advantageous to apply the condensation step according to the invention, which will be discussed later, at least to the stream consisting substantially of gaseous NH3.
In the process according to the invention a condensation step is applied to at least one of the stream consisting substantially of gaseous NH3 or the one or more streams containing NH3, CO2 and H2O supplied to the NH3 separation device. The condensation step can be carried out by means of techniques known per se. Examples of such techniques are: cooling by means of direct contact with a cooling medium and/or by indirect cooling in a heat exchanger and/or contact with a liquid absorbing medium. At least a part of the CO2 that is present is converted to a liquid phase. The liquid phase may already be present during the condensation step, for example because the condensation step is carried out on a gas/liquid mixture; the liquid phase can also be formed during the condensation step, for example because gaseous H2O condenses in which the CO2 as well as NH3 is absorbed; the liquid phase may also be supplied, such as the liquid absorbing medium as mentioned above. Preferably between 40% and substantially all CO2 that is present is brought into a liquid phase; more preferably between 50% and substantially all CO2 that is present is converted to a liquid phase, even more preferably between 75% and 99% or 95%.
The process according to the invention can be applied with the aim of obtaining the stream consisting substantially of gaseous NH3 from the mixture. It may in addition be desirable to also free CO2 and H2O from the mixture, besides NH3. The process according to the invention therefore further preferably comprises, in order to separate CO2 and H2O from the mixture:                a CO2 rectification step, which is applied in a CO2 separation device to the mixture coming from the NH3 separation device while a stream coming from a desorption device is supplied, with a stream consisting substantially of CO2 being formed in the CO2 separation device and being separated from the mixture, and        a desorption step, which is applied in the desorption device to the mixture coming from the CO2 separation device, with a stream consisting substantially of H2O being formed and being separated from the mixture, after which the mixture is returned to the NH3 separation device and/or the CO2 separation device.        
The CO2 rectification step can be carried out with the aid of techniques known per se, in a CO2 separation device. Examples of such a technique are membrane separation and distillation. In the case of distillation the stream consisting substantially of CO2 is the top product. If distillation is applied to the mixture and, as will usually be the case, mainly NH3, CO2 and H2O are present, it is to be expected that account must be taken of the azeotropic behaviour mentioned earlier. The composition in the CO2 separation device, this being the device in which the CO2 rectification step is carried out, must be in the CO2-rich range at the prevailing pressure. If the composition of the mixture supplied from the NH3 separation device, also taking into account the composition of the stream coming from the desorption step, will result in the composition in the CO2 separation device being outside the CO2-rich range, an additional measure is necessary. Examples of such additional measures are: a change in pressure, for example a pressure increase, and/or a change in composition, for example by supplying an additional stream such as an H2O stream. If a pressure increase is applied the pressure in the CO2 separation device usually lies between 0.5 and 10 MPa, more preferably between 1 and 6 MPa and in particular between 1.5 and 5 MPa. The top temperatures in the CO2 separation device then usually lie between 30 and 175° C., preferably between 100 and 150° C., the bottom temperatures usually between 100 and 250° C., preferably between 150 and 200° C.
As indicated above, from the CO2 rectification step a stream consisting substantially of CO2 is released. In addition the mixture is released; the mixture is removed from the CO2 separation device and subsequently supplied to the desorption device where the desorption step is carried out. The aim of the desorption step is to free a stream consisting substantially of H2O from the mixture. This can take place with the aid of techniques known per se, such as with distillation, in which case the stream consisting substantially of H2O is the bottom product. After a stream consisting substantially of H2O has been separated from the mixture in the desorption step, the remaining quantity of the mixture, which still contains NH3, CO2 and H2O, is returned to the NH3 separation device and/or the CO2 separation device.
In this embodiment the condensation step according to the invention is carried out on the stream consisting substantially of gaseous NH3 from the NH3 separation device and/or on at least a part of the stream that comes from the desorption device and that is supplied to the NH3 separation device.
In a special embodiment of the invention the desorption step is carried out in two desorption zones, one zone being operated at a pressure that is almost equal to the pressure in the NH3 separation device and the second one at a pressure that is almost equal to the pressure in the CO2 separation device. The streams leaving the desorption zones are transferred to the two separation devices at the practically corresponding pressures. It was found that this can yield a reduction in steam consumption.
If the condensation step according to the invention is applied to the stream consisting substantially of gaseous NH3, this is done preferably in a submerged condenser with an aqueous stream and/or liquid NH3 being supplied as absorbing medium. A submerged condenser is known per se, for example from NL 8400839 A. The aqueous stream consists substantially of water but may in addition also contain other compounds; examples are NH3, CO2, ammonium carbamate, melamine or urea. In the submerged condenser the stream consisting substantially of gaseous NH3 comes into direct contact with the also supplied aqueous stream and/or liquid NH3, in which CO2 will absorb. This has the advantage that less stringent requirements are specified for the CO2 removal in the NH3 separation device than in the known process, which enhances the operational stability and reduces the risk of solids being formed. As stated earlier, the formation of solids leads to steam consumption that is occasionally and/or structurally higher. Also, through the choice of the feed streams and their temperatures, optimum heat transfer and mass transfer conditions can be chosen, which is especially favourable as regards the transition of CO2 from the gas phase to the liquid phase in the submerged condenser.
If the stream consisting substantially of gaseous NH3, after leaving the submerged condenser and as a result of the contact with the aqueous stream, contains an undesirable quantity of H2O, preferably an absorption step is applied to the stream consisting substantially of gaseous NH3, in which said stream is brought into contact with liquid NH3. As a result, the H2O will be absorbed in the liquid NH3. The absorption step can be carried out with the aid of techniques known per se, for example in a plate column.
In another embodiment of the condensation step according to the invention this step is carried out as a partial condensation step on the stream that comes from the desorption device and that is supplied to the NH3 separation device. The partial condensation step is preferably carried out by means of indirect cooling with a cooling medium, in for example a heat exchanger. The stream coming from the desorption device also contains H2O and NH3; as a result of the partial condensation step at least a part of the H2O will become liquid, in which a part of the CO2 is absorbed as well as a part of the NH3. As a result, operation of the NH3 separation device becomes simpler, and more stable on account of a smaller risk of solids being formed. Preferably the mixture present in the NH3 separation device is used as cooling medium in the partial condensation step.
The first digit of the numbers in the figures is the same as the number of the figure. If the last two digits of the numbers of different figures agree, the parts are the same.